Microfabrication technology that employed the techniques originally developed for the integrated circuit (IC) industry has become popular in diverse areas of science and engineering to create miniaturized transducers. A transducer is a conduit for transforming energy between two or more domains such as mechanical, electrical, thermal, chemical and magnetic. Capacitive micromachined ultrasonic transducers (CMUTs) relate electrical and mechanical domains in energy transfer to transmit and receive ultrasound. As an alternative to piezoelectric transducers, CMUTs offer several advantages such as wide bandwidth, ease of large array fabrication and potential for integration with electronics. Parasitic energy coupling, or crosstalk, between neighboring elements has been observed in immersed operation. It has been determined that the main crosstalk mechanism is a dispersive guided mode propagating in the fluid-solid interface. This coupling degrades the performance of transducers in immersion for medical applications such as diagnostic imaging and high intensity focused ultrasound (HIFU) treatment.
Experimental, analytical and finite element methods have been used to understand the causes and effects of crosstalk in CMUTs. Attempts have been made to reduce the crosstalk, such as changing the substrate thickness and placing etched trenches or polymer walls between the array elements. These efforts were explored using finite element methods. These methods did not significantly affect the crosstalk observed to be −22 dB in immersion.
Another attempt, based on a mathematical CMUT model, covered the top of the array with a thin, lossy solid layer was found to damp out the unwanted resonances that occur on certain frequencies and steering angles due to the coupling in the acoustic medium. However the problem of reducing the dispersive guided mode of an ultrasonic signal remained unaddressed.
Accordingly, there is a need to develop a CMUT array that has reduced crosstalk between the neighboring array elements. There is a further need to improve transducer performance for applications such as diagnostic imaging and high intensity focused ultrasound (HIFU) treatment in medicine. A need exists to reduce the effective element aperture and the ringdown time of a transducer, and improve angular response and range resolution. Further, it would be considered an innovative step with CMUT arrays to improve the axial resolution and bright patterns in the near field.